Gear mechanisms, such as a trapezoidal threaded worm gear mechanism or a rack and pinion gear mechanism as a mechanism, have been used in various kinds of driving sections to convert a rotary motion of an electric motor to an axial linear motion in an electric linear actuator. These motion converting mechanisms involve sliding contact portions. Thus, power loss is increased and this requires an increase in the size of the electric motors and the power consumption. Accordingly, the ball screw mechanisms have been widely adopted as more efficient actuators.
In an electric linear actuator of the prior art, an output member, connected to a nut, can be axially displaced by rotationally driving a ball screw shaft, forming a ball screw, with the use of an electric motor supported on a housing. Since friction of the ball screw mechanism is very low, the ball screw shaft tends to be easily reversely rotated when a pushing thrust load is applied to the output member. Accordingly, it is necessary to hold the position of the output member when the electric motor is stopped.
Accordingly, an electric linear actuator has been developed where a brake mechanism is arranged for an electric motor or a low efficient mechanism such as a worm gear is provided as a power transmitting mechanism. FIG. 7 illustrates one representative example. It includes a ball screw mechanism 53 with a ball screw shaft 51 rotationally driven by an electric motor (not shown). A ball screw nut 52 engages the ball screw shaft 51, via balls (not shown). When a motor shaft (not shown) of the electric motor is rotated, the ball screw shaft 51 connected to the motor shaft is rotated. This moves the ball screw nut 52 in a linear motion (left and right direction in FIG. 7).
The ball screw shaft 51 is rotationally supported on cylindrical housings 54, 55 via two rolling bearings 56, 57. These rolling bearings 56, 57 are secured by a rotation preventing member 59 to prevent loss of the bearings 56, 57, via a securing lid 58.
The ball screw shaft 51 is formed on its outer circumference with a helical screw groove 51a. The groove 51a engages a cylindrical ball screw nut 52 via balls. The ball screw nut 52 is formed on its inner circumference with helical screw groove 52a. The nut 52 has a larger diameter portion 60 on its end.
A side surface of the larger diameter portion 60 is formed with a flat portion 61 cut out. The cut out has a flat end face. A cam follower (rotation preventing means) 62 projects radially outward from a substantially central portion of the flat portion 61.
Since the cam follower 62 is fit in the cut-out portion, the ball screw nut 52 is not able to rotate along with the rotation of the ball screw shaft 51. In addition, since the cam follower 62 rotationally slides against the cut-out portion, it is possible to reduce problems with sliding friction and wear of the housing (see e.g. Patent Document 1, JP2007-333046 A).
In the prior art electric linear actuator 50, the use of the cam follower 62 as a ball screw nut 52 rotation preventing mechanism makes it possible to reduce problems with sliding friction and wear of the housing of the actuator. However, since the cam follower 62 itself uses a rolling bearing, it is believed that the manufacturing cost of the electric linear actuator will be increased. In addition, any anti-wear mechanism will be required when the housing is made of aluminum material.